From TheBestLinks.com
A signal is an asynchronous event transmitted between one process and another (in Linux, UNIX and other POSIX-compliant operating systems, and also in several real-time operating system). The kill() system call is used to send signals, and the signal() system call is used to set up signal handlers—functions to call when a process receives a signal.
From the man page:
SIGNAL(7) Linux Programmer's Manual SIGNAL(7)
NAME
signal - list of available signals
DESCRIPTION
Linux supports both POSIX reliable signals (hereinafter
"standard signals") and POSIX real-time signals.
Standard Signals
Linux supports the standard signals listed below. Several
signal numbers are architecture dependent, as indicated in
the "Value" column. (Where three values are given, the
first one is usually valid for alpha and sparc, the middle
one for i386, ppc and sh, and the last one for mips. A -
denotes that a signal is absent on the corresponding
architecture.)
The entries in the "Action" column of the table specify
the default action for the signal, as follows:
Term Default action is to terminate the process.
Ign Default action is to ignore the signal.
Core Default action is to terminate the process and dump
core.
Stop Default action is to stop the process.
First the signals described in the original POSIX.1 stan
dard.
Signal Value Action Comment
-------------------------------------------------------------------------
SIGHUP 1 Term Hangup detected on controlling terminal
or death of controlling process
SIGINT 2 Term Interrupt from keyboard
SIGQUIT 3 Core Quit from keyboard
SIGILL 4 Core Illegal Instruction
SIGABRT 6 Core Abort signal from abort(3)
SIGFPE 8 Core Floating point exception
SIGKILL 9 Term Kill signal
SIGSEGV 11 Core Invalid memory reference
SIGPIPE 13 Term Broken pipe: write to pipe with no readers
SIGALRM 14 Term Timer signal from alarm(2)
SIGTERM 15 Term Termination signal
SIGUSR1 30,10,16 Term User-defined signal 1
SIGUSR2 31,12,17 Term User-defined signal 2
SIGCHLD 20,17,18 Ign Child stopped or terminated
SIGCONT 19,18,25 Continue if stopped
SIGSTOP 17,19,23 Stop Stop process
SIGTSTP 18,20,24 Stop Stop typed at tty
SIGTTIN 21,21,26 Stop tty input for background process
SIGTTOU 22,22,27 Stop tty output for background process
The signals SIGKILL and SIGSTOP cannot be caught, blocked,
or ignored.
Next the signals not in the POSIX.1 standard but described
in SUSv2 and SUSv3 / POSIX 1003.1-2001.
Signal Value Action Comment
-------------------------------------------------------------------------
SIGBUS 10,7,10 Core Bus error (bad memory access)
SIGPOLL Term Pollable event (Sys V). Synonym of SIGIO
SIGPROF 27,27,29 Term Profiling timer expired
SIGSYS 12,-,12 Core Bad argument to routine (SVID)
SIGTRAP 5 Core Trace/breakpoint trap
SIGURG 16,23,21 Ign Urgent condition on socket (4.2 BSD)
SIGVTALRM 26,26,28 Term Virtual alarm clock (4.2 BSD)
SIGXCPU 24,24,30 Core CPU time limit exceeded (4.2 BSD)
SIGXFSZ 25,25,31 Core File size limit exceeded (4.2 BSD)
Up to and including Linux 2.2, the default behaviour for
SIGSYS, SIGXCPU, SIGXFSZ, and (on architectures other than
SPARC and MIPS) SIGBUS was to terminate the process (with
out a core dump). (On some other Unices the default
action for SIGXCPU and SIGXFSZ is to terminate the process
without a core dump.) Linux 2.4 conforms to the POSIX
1003.1-2001 requirements for these signals, terminating
the process with a core dump.
Next various other signals.
Signal Value Action Comment
--------------------------------------------------------------------
SIGIOT 6 Core IOT trap. A synonym for SIGABRT
SIGEMT 7,-,7 Term
SIGSTKFLT -,16,- Term Stack fault on coprocessor (unused)
SIGIO 23,29,22 Term I/O now possible (4.2 BSD)
SIGCLD -,-,18 Ign A synonym for SIGCHLD
SIGPWR 29,30,19 Term Power failure (System V)
SIGINFO 29,-,- A synonym for SIGPWR
SIGLOST -,-,- Term File lock lost
SIGWINCH 28,28,20 Ign Window resize signal (4.3 BSD, Sun)
SIGUNUSED -,31,- Term Unused signal (will be SIGSYS)
(Signal 29 is SIGINFO / SIGPWR on an alpha but SIGLOST on
a sparc.)
SIGEMT is not specified in POSIX 1003.1-2001, but neverth
less appears on most other Unices, where its default
action is typically to terminate the process with a core
dump.
SIGPWR (which is not specified in POSIX 1003.1-2001) is
typically ignored by default on those other Unices where
it appears.
SIGIO (which is not specified in POSIX 1003.1-2001) is
ignored by default on several other Unices.
Real-time Signals
Linux supports real-time signals as originally defined in
the POSIX.4 real-time extensions (and now included in
POSIX 1003.1-2001). Linux supports 32 real-time signals,
numbered from 32 (SIGRTMIN) to 63 (SIGRTMAX). (Programs
should always refer to real-time signals using notation
SIGRTMIN+n, since the range of real-time signal numbers
varies across Unices.)
Unlike standard signals, real-time signals have no prede
fined meanings: the entire set of real-time signals can be
used for application-defined purposes. (Note, however,
that the LinuxThreads implementation uses the first three
real-time signals.)
The default action for an unhandled real-time signal is to
terminate the receiving process.
Real-time signals are distinguished by the following:
1. Multiple instances of real-time signals can be queued.
By contrast, if multiple instances of a standard sig
nal are delivered while that signal is currently
blocked, then only one instance is queued.
2. If the signal is sent using sigqueue(2), an accompany
ing value (either an integer or a pointer) can be sent
with the signal. If the receiving process establishes
a handler for this signal using the SA_SIGACTION flag
to sigaction(2) then it can obtain this data via the
si_value field of the siginfo_t structure passed as
the second argument to the handler. Furthermore, the
si_pid and si_uid fields of this structure can be used
to obtain the PID and real user ID of the process
sending the signal.
3. Real-time signals are delivered in a guaranteed order.
Multiple real-time signals of the same type are deliv
ered in the order they were sent. If different real-
time signals are sent to a process, they are delivered
starting with the lowest-numbered signal. (I.e., low-
numbered signals have highest priority.)
If both standard and real-time signals are pending for a
process, POSIX leaves it unspecified which is delivered
first. Linux, like many other implementations, gives pri
ority to standard signals in this case.
According to POSIX, an implementation should permit at
least _POSIX_SIGQUEUE_MAX (32) real-time signals to be
queued to a process. However, rather than placing a per-
process limit, Linux imposes a system-wide limit on the
number of queued real-time signals for all processes.
This limit can be viewed and (with privilege) changed via
the /proc/sys/kernel/rtsig-max file. A related file,
/proc/sys/kernel/rtsig-nr, can be used to find out how
many real-time signals are currently queued.
CONFORMING TO
POSIX.1
BUGS
SIGIO and SIGLOST have the same value. The latter is com
mented out in the kernel source, but the build process of
some software still thinks that signal 29 is SIGLOST.
SEE ALSO
kill(1), kill(2), setitimer(2), sigaction(2), signal(2),
sigprocmask(2), sigqueue(2)
Related links
Top visited
0 of
0 links
[no links posted yet]
>> place link >>
Discussion
Last posted
0 of
0 messages
[no messages posted yet]
>> post message >>
Watch
You can
add this article to your own "watchlist" and receive e-mail notification about all changes in this page.
